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CHEMISTRY > 4.4 MOLES I > Flashcards

4.4 MOLES I Flashcards

1
Q

Define Relative Atomic Mass

A
  • The weighted average mass of the isotopes of an element
  • It is measured on a scale on which a carbon-12 atom has exactly 12 unit
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2
Q

Define Relative Molecular Mass

A
  • The total average mass of one molecule of a compound
  • Atoms which are compared to 1/12th of one atom of Carbon-12
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3
Q

Symbol for Relative Atomic Mass

A

Ar

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4
Q

Symbol for Relative molecular mass

A

Mr

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5
Q

How do you work out the Mr of a substance

A
  1. If a word compound, find the formulae
  2. Break each atom in the substance down to it’s Ar
  3. Add up the ammounts
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6
Q

Unit for Ar/Mr

A

No unit

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7
Q

What is Avogadro’s constant

A

6.022 x10^23 atoms/molecules

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8
Q

Symbol for Avogadro’s constant

A

NA

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9
Q

How to calculate the number of molecules in a certain amount of a substance

A

Amount of any substance (in moles) x Avogadro’s Constant

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10
Q

Equation for composition of substances

A

% of Substance = Ar of substance / Mr of Compound x100

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11
Q

What should be done before the equation if working out the composition of a substance that is a word formulae

A

Convert it into a molecular formula

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12
Q

What does a mole represent

A

How many particles of substance there are

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13
Q

Formulae for Moles

A

Moles = mass (g) / Ar or Mr

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14
Q

Formulae for mass (g)

A

Mass (g) = Moles x Mr or Ar

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15
Q

What must you do to a substance where the mass ISN’T in grams

A

Convert it FIRST

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16
Q

What is the empirical formulae

A

It gives the simplest whole number ratio of atoms

17
Q

How to work out the empirical formula

A
  1. Take the mass/percentage and divide by the Ar of the element you are finding (finding the moles)
  2. Divide each amount by the lowest answer from the previous step
  3. Write as a ratio then put into formula layout
18
Q

What must you never do when calculating empirical formula

A

Round up or down

19
Q

When calculating empirical formula, can you use fractions in steps

A

No - Always just use decimals

20
Q

What does molecular formula show

A

Actual number of atoms in a molecule

21
Q

How to calculate molecular formula

A
  1. Calculate Mr of the substance with the empirical formula
  2. Divide the RFM by the Mr of the substance (previously worked out)
  3. Multiply the substances in the empirical formula by this number
22
Q

What is water of crystallisation

A

When salt crystals are hydrated and the water is trapped inside

23
Q

How to find the amount of water in a water of crystallisation question

A
  1. Calculate the mass of water lost (Starting mass - final mass = mass of water)
  2. Calculate moles of the salt (Use mass of salt ONLY and divide by Mr of salt ONLY)
  3. Do the same with the water
  4. Divide the salt value and water value by the smallest value
  5. Use the ratio to find the amount of water
24
Q

How to find the mass of a substance needed to react with an amount of another substance (reacting masses)

A
  1. Convert mass into moles of the substance we know the mass of
  2. Use the ratio from the balancing numbers to find the moles of the substance we need
  3. Convert that into mass using mass=moles/Mr
25
Q

What is a limiting substance

A

when there isn’t enough of a substance

26
Q

What is a substance in excess

A

When there is more than enough for that reaction

27
Q

Determine which is limiting and which is excess in : 10g of Mg and 10g HCl in Mg + 2HCl → MgCL2 + H2

A
  1. Calculate the moles of Mg
  2. Use the ratio from the balancing to find moles of the other substance
  3. Find the mass of that substance
  4. The HCl is limiting as 30.4g is needed whilst we only have 10
  5. Therefore Mg is in excess
28
Q

How to work out how many moles of a product is formed when you have the moles of the products

A
  1. Balance equation
  2. Divide number of moles of the products by the balancing number
  3. Multiply the smallest value by the balancing number of the product
29
Q

Why is percentage yield never %100

A
  1. Loss of substances
  2. Unreacted material
  3. Impure substances
30
Q

Percentage yield equation

A

Percentage yield = Experimental Yield / Theoretical Yield x100

31
Q

How to calculate the percentage yield if given masses

A
  1. Find a balanced equation
  2. Find the moles of the product
  3. Use the ratio of balancing numbers to then find moles of the product
  4. Find mass of product
  5. Use %yield equation with mass from calculation and from question

CHEMISTRY (17 decks)

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